“A Dark Song” is an Irish horror film with a modest name that suggests a momentary swerve into blackness rather than the committedly bleak, untrustworthy doings in Liam Gavin’s confidently moody first feature.

Grim-faced Sophia (Catherine Walker) signs a lease on a desolate estate in northern Wales, then sternly interviews an equally serious, red-bearded hulk in a barn jacket named Joseph (Steve Oram) for a dangerous black magic assignment. She’s a bereft mother desperate to invoke a guardian angel so she can make contact with her murdered son, and he’s a hot-tempered, alcoholic occultist with a drill sergeant mentality about hewing to ritual when the byproduct of demons might be involved.

Alone together in her remote house, shut off from the world for months and with plenty of candles and chalk circles, this is the opposite of “meet cute.” “A Dark Song” isn’t exactly an entertaining dive into the types of invocation vigils that have made nervy, full-bodied classics out of “The Haunting” and “The Conjuring.”

Gavin’s specialties are emotional bitterness about the cruelty of the world, light-challenged atmospherics (Cathal Watters’ cinematography is evocative), and intense detail about the dos and don’ts of dark-arts ceremonies. The problem is that the ritual mumbo jumbo — and Joseph’s browbeating methods to get Sophie to stop being impatient and just do what he says — gets old after a while, and wondering whether anything will come of it loses its tension as well.

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That’s because Gavin hasn’t written a story so much as created an environment for two shut-down people and assumed that director-enhanced mood and pockets of excitement — drinking blood, a near-drowning in a bathtub, a struggle with a knife nearby — will keep us going until the is-it-real-or-not finale. (Having a blackbird hit a window, then Joseph intone, “It’s begun,” after which nothing begins, is just wrong.) Even a tiny hint of added friction introduced when Sophie reveals a key detail about her boy’s killing doesn’t generate any new suspense.

The two leads are resolute soldiers about it all, but they’re dutifully edgy elements in a stylist’s frame instead of fully realized characters living out what is supposed to be the riskiest time of their lives. That’s why it’s equally difficult to see the big ending as anything other than a magician’s ta-da, rather than an emotional payoff.